In recent years, owing to the spread of the Internet, the traffic volume of a backbone communication system is increased rapidly, and thus an early practical use of an ultra-high speed optical communication system beyond 40 Gbps is highly expected.
As to a technology realizing the ultra-high speed optical communication system, a polarization multiplexing-demultiplexing technology has attracted considerable interest. In the polarization multiplexing-demultiplexing technology, an optical transmitter multiplexes two optical signals having the same frequency band of carrier waves and polarization states orthogonal to each other. Hereinafter, such an optical signal obtained by multiplexing two optical signals having the same frequency band of carrier waves and polarization states orthogonal to each other is called a polarization multiplexed optical signal. And an optical receiver receives the transmitted polarization multiplexed optical signal from the optical transmitter and then demultiplexes the received polarization multiplexed optical signal into two optical signals. And thus a double transmission speed is realized.
An example of such technology related to a polarization multiplexing transmission system in which two optical signals having polarization states orthogonal to each other propagate through the same fiber is disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-528175. In this related art, an optical receiver separates two components with orthogonal polarizations included in the received signal by filtering. At that time, the related optical receiver is operable to filter them in accordance with an appropriate transfer matrix which is dynamically controlled on the basis of the output signals in such a manner as to approximate the reverse transfer matrix of the fiber in the region of the spectrum occupied by the optical signals at the same wavelength with orthogonal polarizations. As a result, the related receiver compensates for polarization mode dispersion and polarization rotation introduced by the fiber, eliminates distortion and mutual interference effects for both of the signals, and outputs demultiplexed optical signals corresponding to the two transmitted signals.